We measured plasma leptin concentration by radio-immunoassay in 204 normal-weight and obese subjects aged 18-80 using full-length recombinant human leptin as a standard. Fasting levels between 1.2 and 97.9 ng/ml were observed. Plasma leptin concentration was highly correlated with percent body fat (r=0.710, p<0.0001) and was 3 times as high in women as in men . Circulating leptin was inversely related to age and was reduced 53% in subjects over age 60. A statistical model containing percent body fat, gender, and age accounted for 65% of the variance in plasma leptin levels. Glycerol release from fat cells can be studied by stable isotope dilution mass spectrometry and is a marker of lipolysis. Leptin was not independently related to abdominal fat distribution or to plasma lipids and lipoproteins, chronic energy intake, diet composition, plasma insulin or maximum oxygen consumption. However, plasma leptin was reduced by 26% in 5 obese subjec ts who consumed a 1000 calorie diet for 10 days (p=0.004). We conclude that circulating leptin rises continuously with increasing adiposity. Gender, age, and short-term caloric restriction may be important secondary regulators of plasma leptin.